This is related to #1369. This should be fixed in 2.5.2b1, as most of the stat implementation (which isfile relies on) was rewritten. Reopen this ticket if you can still reproduce it on 2.5.2, but I can't on OSX
Jython 2.5.2b1 (trunk:7095M, Sep 1 2010, 22:35:18)
[Java HotSpot(TM) 64-Bit Server VM (Apple Inc.)] on java1.6.0_17
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>>> import os
>>> os.path.isfile(None)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/Users/pjenvey/src/java/jython/dist/Lib/posixpath.py", line 208, in isfile
st = os.stat(path)
File "/Users/pjenvey/src/java/jython/dist/Lib/posixpath.py", line 208, in isfile
st = os.stat(path)
TypeError: coercing to Unicode: need string or buffer, NoneType found